Di-chlorinated benzothiadiazole unit: a new strong electron-withdrawing ability acceptor toward fast-switching green low band gap D-A electrochromic polymers

Di-halogenated benzothiadiazole (BT) is one of the potential acceptor units in constructing low band gap d-A polymers, but it has not attracted widespread attention, especially for the di-chlorinated BT unit. Herein, in this work, two kinds of d-pi-A-pi-D electrochromic conjugated precursors with di-halogenated BT (5,6-dichlorobenzo [c][1,2,5]thiadiazole and 5-chloro-6-fluorobenzo[c][1,2,5]thiadiazole) as the acceptor unit, 3-dodecyl thiophene as the pi bridge, and 3,4-ethylenedioxythiophene (EDOT) as the donor unit, were designed and synthesized, and their corresponding d-A polymers were successfully obtained by electrochemical deposition method. The photophysical study proves that introducing more chlorine atoms among the precursors will lead to stronger steric hindrance, lower HOMO energy level, blue-shifted absorption spectra, lower quantum yield, but larger Stokes shift. Both of them show very low oxidation potential, which is beneficial for them to obtain high-quality d-A polymers. The introduction of di-chlorinated BT unit obviously improved the polymers' electroactivity and redox stability, and the remained activity of P(ClCl) was above 74.79 after 1000 cycles. The optical band gap of P (ClCl) is further reduced to 1.28 eV, and the reversible transition from the neutral dark green to the oxidized dark blue can be realized in the redox process. As a result, various favorable electrochromic parameters for P(ClCl) were achieved, such as the optical contrast in the near-infrared region is 27.23 %, the response time is as low as 0.4 s, and the coloration efficiency is as high as 151.3 cm2 C-1, as well as the good optical stability and satisfied memory effect. Overall, the subtle change of halogen atoms in BT units is a very efficient method to modulate the photophysical properties of d-pi-A-pi-D precursors, also providing good guidelines in designing highly efficient d-A electrochromic polymers.